1. Technical Field
This invention relates to an optical semiconductor device and particularly, but not exclusively, to a semiconductor laser device that emits variable radiation in the wavelength range 630 nm to 680 nm. The laser device may be of the edge-emitting or of the surface-emitting type.
2. Background Art
Laser devices or laser diodes (LDs) fabricated in the (Al,Ga,In)P material system which emit visible light in the 630 nm-680 nm wavelength range are becoming increasingly important components of professional and consumer products. For example, it is envisaged that the Digital Video Disc (DVD) system will enjoy a 635 nm-650 nm wavelength LD capable of delivering up to 30 mW output power up to a temperature of 60xc2x0 C. The next generation of semiconductor lasers will need an even greater maximum power output up to a higher (eg. 70xc2x0 C.) operating temperature.
By the (Al,Ga,In)P system is meant the family of compounds having the general formula (AlxGa1-x)1-yInyP, where both x and y are between 0 and 1. One particular advantage of this semiconductor system is that it is lattice-matched to a GaAs substrate when the indium mole fraction, y, is equal to 0.48.
A principal limitation of current (Al,Ga,In)P laser diodes is that they are incapable of operating for long periods (or with a sufficiently low threshold current) at the highest specified operating temperature. It is generally believed that this is caused by electron leakage from the active region of the device into the surrounding optical guiding region and subsequently into the p-type cladding region.
One type of laser device is the separate confinement heterostructure laser. The generic structure of a separate confinement laser structure intended to generate light at 630-680 nm will now be described with reference to FIGS. 1 and 2.
Curve (a) of FIG. 1 illustrates the difference between the xcex93-conduction band energy of (AlxGa1-x)0.82In0.48P and Ga0.52In0.48P, as a function of the aluminium mole fraction in the quaternary alloy. Curves (b) and (c) of FIG. 1 show the difference between the X-conduction band energy and the xcex93-valance band energy respectively. FIG. 1 assumes that the bandgap difference between (Al,Ga)InP and GaInP is split in a ratio of 70:30 between the conduction band offset and the valance band offset.
It will be seen that the minimum energy in the conduction band of (Al,Ga,In)P is a function of the aluminium content. There is a crossover from a xcex93-band minimum to an X-band minimum at an aluminium concentration of about 0.55.
The terms xcex93-band and X-band as used herein refer to symmetry points in the Brillouin zone and are standard terms in solid state physics, see for example R. A. Smith xe2x80x9cSemiconductorsxe2x80x9d, (Cambridge University Press, 1978). The terms xcex93-minimum and X-minimum refer to the minimum energy level of the xcex93-band and the X-band, respectively.
FIG. 2 is a schematic band structure of a separate confinement laser structure fabricated in the (Al,Ga,In)P system. It consists of an n-doped (Al0.7Ga0.3)0.52In0.48P cladding region 1, an (Al0.5Ga0.5)0.52In0.48P optical guiding region 2, 4, a GaInP quantum well active region 3 disposed within the (Al0.8Ga0.5)0.52In0.48P optical guiding region, and a p-doped (Al0.7Ga0.2)0.52In0.48P cladding regions. Optical transitions giving rise to laser action in the quantum well active region 3 of the laser diode originate from xcex93-electrons in the GaInP quantum well active region.
The electron leakage current consists of that fraction of the electrons which have sufficient thermal energy to surmount the potential barrier on the right hand side of FIG. 2, and pass into the p-doped cladding region 5. It will be seen that xcex93-electrons are confined in the optical guiding region (waveguide region) by a potential barrier of only around 90 meV at the interface with the p-doped cladding region. This relatively small barrier height allows a significant proportion of electrons to escape. Moreover, holes in the valence band are confined only by a potential barrier of around 50 meV, and this low barrier height also allows significant carrier escape. Furthermore the X-conduction band in the p-cladding region 5 is some 50 meV below the xcex93-cladding band in the waveguiding region 2, 4, and this allows electrons to escape from the wavelength region 2, 4 through the X-states in the p-doped cladding regions. Thus, the laser illustrated in FIG. 2 has a high leakage current, and so has poor performance at high temperatures.
P. M. Smowton et al. in xe2x80x9cApplied Physics Lettersxe2x80x9d Vol. 67, pp. 1265-1267 (1995) show that an important leakage mechanism for electrons can be via the indirect X-valley of the conduction bands in the p-side guiding and cladding regions of a separate confinement hetero-structure laser having two Ga0.41In0.59P quantum wells separated by a barrier, or set in an optical guiding region of (AlyGa1-y)0.51In0.49P (where y is variously 0.3, 0.4 and 0.5), and clad with (Al0.7Ga0.3)0.51In0.49P cladding regions, doped with Zn on the p-side and Si on the n-side. However, no proposals are made for mitigating the problems caused by loss of electrons via this mechanism.
There have been a number of proposals to improve the temperature performance of laser devices fabricated in the (Al,Ga,In)P system.
T. Takagi et al., xe2x80x9cIEEE Journal of Quantum Electronics)xe2x80x9d Vol. 27, No. 6, 1511 (1991) have proposed introducing a multiple-quantum well barrier in the cladding region.
In UK Patent Application No. 9526631.8, it is proposed that the insertion of a xcex4-doped p-type layer in the p-doped cladding region of a SCH laser diode will have the effect of increasing the band bending on the p-side of the hetero-junction and thus increase the potential barrier which is presented to thermal leakage of electrons.
G. Hatakoshi et al., xe2x80x9cIEEE Journal of Quantum Electronics, Vol. 27, p1476 (1991) have proposed increasing the doping level of the p-doped cladding region in order to increase the potential barrier between the waveguiding region and the p-doped cladding region. UK Patent Application No. 9626644.0 discloses a semiconductor laser which incorporates an electron reflecting layer, to prevent X-electrons escaping into the p-doped cladding region. UK Patent Application No. 9626657.2 discloses the use of electron capture layers to capture X-electrons, and transfer them to a xcex93-confined energy level in the active region. However, the affectiveness of these schemes to improve the temperature characteristics of an (Al,Ga,In)P laser device is currently unclear.
The principle of operation of a multiple quantum well barrier (MQB) is to incorporate an MQB in the p-type cladding region of an SCH laser device. The MQB consists of very thin alternating layers of, for example, (In,Ga,)P and (Al,Ga,In)P (for an (Al,Ga,In)P laser). An electron which has sufficient thermal energy to escape from the SCH structure will be quantum mechanically reflected at each of the interfaces of the MQB. If the layer thicknesses are chosen to be xcex/4 in thickness, where xcex is the electron wavelength, then a band of energies can be engineered at which electrons will be reflected with a probability of 1. Almost unity reflectivity of the electrons can be engineered to exist well above the classical barrier height. Theoretically, a MQB can increase the effective barrier height by up to a factor of 2 compared to the classical barrier height.
K. Kishino et al. xe2x80x9cApplied Physics Lettersxe2x80x9d Vol. 58, pp. 1822-1824 (1991) and H. Hamada et al xe2x80x9cElectronics Lettersxe2x80x9d Vol. 28, pp 1834-1836 (1992) provide evidence to show that the temperature dependence of the threshold current of shot wavelength lasers is improved through the use of such reflectors. However, the effectiveness of the reflectors is usually inferred from LD operating characteristics rather than from a direct measurement of the enhancement of the barrier height. It is difficult to quantify, therefore, just what advantage has accrued from the use of a MQB in comparison to any advantage that might have occurred simply due to better processing or better medical quality. Furthermore, it should be noted that the effectiveness of the MQB is realised only if the coherence length of the electrons is long. Anything which destroys this coherence such as, for example, interface scattering, will significantly diminish the reflectivity properties.
Increasing the doping level of the p-doped cladding layer will increase the potential barrier between the waveguiding region 4 and the p-doped cladding region 5. However, there are practical limitations to the amount of p-doping which can be incorporated into (Al,Ga,In)P or (Al,In)P cladding regions. This is particularly true of MOCVD grown materials, where a maximum impurity concentration of approximately 2xc3x971018cmxe2x88x923 can be achieved using either Zn or Mg. An example of this is given by D. P. Bour et al. in
xe2x80x9cIEEE Journal of Quantum Electronicsxe2x80x9d Vol. 30, pp. 593-606 (1994). However, any further increase in the dopant concentration using this technique causes the dopant to diffuse into the active region of the device, thereby degrading its performance.
It is possible to increase the aluminium content of the cladding layer 5 in order to increase the potential barrier between the waveguiding region 4 and the p-doped cladding region 5, and thereby increase the xcex93-electron and valance band hole confinement. This approach is illustrated in FIG. 3. This illustrates an SCH laser structure that is similar to that shown in FIG. 2, but in which the cladding regions 1,5 are formed of AlInP. The potential barrier between the optical guiding region 4 and the p-doped cladding region 5 is now 250 meV, and the potential barrier confining the valence band holes is now 100 meV. Thus, the laser structure shown in FIG. 3 will have improved carrier confinement compared to the structure shown in FIG. 2.
Increasing the aluminium content of the cladding layers 1,5 will not, however, prevent carrier escape via the X-band states in the p-doped cladding region 5.
A first aspect of the present invention provides an optical semiconductor device including: an active region; and a p-doped cladding region disposed on one side of the active region; wherein an electron-reflecting barrier is provided at the p-side of the active region for reflecting xcex93-electrons and X-electrons, the electron-reflecting barrier providing a greater potential barrier to xcex93-electrons than the p-doped cladding region.
S. J. Chang et al., xe2x80x9cIEEE Photonics Technology Lettersxe2x80x9d Vol. 10, No. 5, p651 (1998) describes in (Al,Ga,In)P laser diode having an emission wavelength of 624 nm. The laser diode is provided with a triple tensile strain barrier cladding layer to introduce a barrier to xcex93-electrons. Improved temperature dependence is observed. However, the tensile reflective layers do not provide any barrier to X-electrons. On the contrary, they introduce quantum wells for trapping X-electrons. Thus, significant carrier loss via the X-band states in the p-doped cladding region still occur with this structure.
U.S. Pat. No. 5,509,024 discloses a laser diode having a tunnel barrier layer. An AlAs barrier layer is introduced between the optical guiding region and the p-doped cladding region to act as a barrier to xcex93-electrons.
U.S. Pat. No. 5,509,204 does not address the problem of carrier loss via the X-states in the p-doped cladding region. The patent proposes locating the AlAs barrier layer between a (Al0.5Ga0.5)0.52In0.48P optical guiding region and a (Al0.7Ga0.3)0.52In0.48P cladding region. At the date of this patent, neither the band offsets not the xcex93-X direct-indirect bandgap changeover in the (Al,Ga,In)P system were well established. In the light of recent experimental evidence concerning the xcex93-X direct-indirect bandgap changeover, it can be seen that the structure proposed in U.S. Pat. No. 5,509,024 will introduce a 0.32 eV trapping quantum well for X-electrons. Thus, while the scheme proposed in this patent will introduce a potential barrier of around 0.58 eV for xcex93-electrons, it will not address the problem of carrier loss via the X-states in the p-doped cladding region. Indeed, the introduction of the 0.32 eV quantum well for X-electrons will aggravate this problem.
In contrast to the above prior art, however, the present invention provides a barrier that will prevent leakage of both xcex93-electrons and X-electrons. The problem of carrier loss via the X-states in the p-doped cladding region is prevented, or at least significantly reduced, since the electron-reflecting barrier reflects X-electrons as well as xcex93-electrons.
The electron-reflecting barrier may include a first electron-reflecting layer for reflecting xcex93-electrons and a second electron-reflecting layer for reflecting X-electrons. This is a convenient way of providing a barrier for both xcex93-electrons and X-electrons.
At least one of the electron-reflecting layers may be a strained layer. In some cases, a strained semiconductor layer has a forbidden bandgap that is greater than the forbidden bandgap of the bulk semiconductor material, and using such a strained layer as an electro-reflecting layer will increase the potential barrier to electron and hole leakage.
One of the electron-reflecting layers may be in a state of compressive strain and the other of the electron-reflecting layers may be in a state of tensile strain. The two electron-reflecting layers will thus form a strain-compensated barrier. It has been reported that a strain-compensated barrier can be made thicker than the sum of the critical thicknesses of the individual layers without introducing defects into the layers. This means that a strain compensated electron-reflecting barrier can be made thicker without introducing defects, and a thicker barrier will reflect more electrons back into the active region thereby improving the confinement of the electrons.
The device may be a light-emitting diode, or it may be a laser device. The laser device may be a separate confinement heterostructure laser device including an optical guiding region, the active region being disposed within the optical guiding region. The layer for reflecting xcex93-electrons may be disposed between the optical guiding region and the layer for reflecting X-electrons. The xcex93-conduction band of the optical guiding region may be substantially degenerate with the X-conduction band of the layer for reflecting xcex93-electrons. This ensures that the layer for reflecting xcex93-electrons does not produce a quantum well for X-electrons.
Alternatively, the layer for reflecting xcex93-electrons may be disposed between the layer for reflecting X-electrons and the p-doped cladding region. In this arrangement, the formation of a quantum well for X-electrons in the layer for reflecting xcex93-electrons does not cause a serious problem, since few X-electrons reach the layer for reflecting xcex93-electrons. This arrangement therefore allows a wider choice of materials for the optical guiding region.
The electron-reflecting barrier may include a plurality of first electron-reflecting layers for reflecting xcex93-electrons and a plurality of second electron-reflecting layers for reflecting X-electrons. The electron reflecting barrier may be a superlattice structure. This is possible because the electron barrier is strain compensated.
The divide may be fabricated in the (Al,Ga,In)P system, the or each layer for reflecting xcex93-electrons may be AlP or GaP, and the or each layer for reflecting X-electrons may be InP. This provides a convenient way of reducing the leakage current in an (Al,Ga,In)P laser.
The layer for reflecting xcex93-electrons may be AlP and the optical guiding region may be (Al0.9Ga0.7)0.52In0.48P. This makes the xcex93-conduction band of the optical guiding region substantially degenerate with the x-conduction band of the layer for reflecting xcex93-electrons, as is preferable when the layer for reflecting xcex93-electrons is disposed between the optical guiding region and the layer for reflecting X-electrons.
The thickness of each of the electron-reflecting layers may be 16A or less. The this thickness is lower than the critical thickness at which the formation of misfit dislocations in a strained layer becomes energetically favourable.
At least one of the electron-reflecting layers may be p-doped. If the electron-reflecting layers are heavily p-doped band banding will occur, and this will increase the height of the potential barrier to electron transport into the p-cladding region. The p-doping will also reduce the barrier height for hole transport into the optical guiding region.
The first electron-reflecting layer, or at least one of the first electron-reflecting layers (if there are more than one), may contain indium. Introducing indium into an AlP or GaP strained layer will reduce the strain in the layer, and will hence increase the critical thickness of the layer. The layer(s) for reflecting xcex93-electrons can therefore be made thicker, and this will reduce the probability that electrons can tunnel through the layer.
The electron-reflecting barrier may be disposed between the optical guiding region and the p-doped cladding region.
A second aspect of the present invention provides an optical semiconductor device including: an optical guiding region; an active region having at least one energy well, said active region being disposed in said optical guiding region; and n-doped and p-doped cladding regions disposed on opposite sides of the optical guiding region; wherein an electron-reflecting layer for reflecting xcex93-electrons is disposed at the p-side of the active region; and wherein the xcex93-conduction band of the optical guiding region is substantially degenerate with the X-conduction band of the electron-reflecting layer.
This aspect of the present invention addresses the problem outlined above with reference to the lasers described in S. J. Chang et al. and in U.S. Pat. No. 5,509,024. In this aspect, the X-conduction band of the electron reflecting layer is chosen to be substantially degenerate with the xcex93-conduction band of the optical guiding region. This prevents the formation of a quantum well for X-electrons in the layer for reflecting xcex93-electrons. This can be done, for example, by selecting the composition of the optical waveguiding region appropriately.
WO 97/40560 discloses an (Al,Ga,In)P light emitting diode. An AlP barrier layer is disposed between the active region and the p-type cladding region of the LED. However, while this barrier layer will increase confinement for xcex93-electrons, a quantum well will be produced in the X-conduction band. The depth of this quantum well will be approximately 0.4 eV and, as explained above, introduction of this quantum well will aggravate the problem of the loss of electrons via the X-states in the p-doped cladding region of the LED.
The optical guiding region may be formed of (Al0.3Ga0.7)0.52In0.48P, and the electron-reflecting layer may be formed of AlP. This is a convenient way of putting the second aspect of this invention into practice in the (Al,Ga,In)P system.
The electron-reflecting layer may be p-doped.
The electron-reflecting layer may be disposed between the optical guiding region and the p-doped cladding region.
The device may be a separate confinement heterostructure laser device.